1. Field of the Invention
The invention relates to thermally alterable compositions and more specifically to coatings which can be switched by imagewise exposure to heat-convertible radiation from a hydrophilic state to a hydrophobic state, especially using a focused infra-red (IR) laser. A main application is lithographic printing masters.
2. Background of the Invention
There is continuing interest in monochrome image-forming media suitable for address by lasers, particularly media requiring no processing subsequent to the laser exposure (xe2x80x98direct writexe2x80x99 media), or requiring only uniform thermal processing to develop the image. Such media do not generate waste materials (e.g., in the form of processing solutions, used donor sheets, strippable cover sheets, and the like) which may present a disposal problem, and are the most convenient media from the user""s point of view.
Two main areas of utility for such monochrome image-forming media are graphic arts films and medical imaging films and papers, which generally impose differing requirements on the imaging media. Graphic arts films are normally used to provide a contact mask for subsequent UV flood-exposure of a printing plate or proofing element. For this reason, they should have a high contrast, strong absorption in the UV in image areas, and high UV transparency in the background areas. The visual appearance (tone) of the graphic arts image is less important. On the other hand, medical imaging media are used to record on film or paper the output of digital radiography equipment, CAT scanners, magnetic resonance scanners, ultrasound scanners etc. To facilitate inspection and interpretation of the images by the human eye, continuous tone images with a neutral black appearance are required, preferably with a high Dmax capability e.g., greater than 3.0).
In view of these contradictory requirements, different types of imaging media have been proposed for the different applications. For example, the high contrast requirements of graphic arts media are most easily met by methods such as mass transfer, ablation transfer or peel-apart systems, as described in U.S. Pat. Nos. 3,962,513, 5,171,650, 5,352,562, 4,981,765 and 5,262,275, EP-A-0465727 and EP-A-0488530, and International Patent Applications Nos. WO90/12342, WO93/04411, WO93/03928 and WO88/04237. Such methods generally involve the disposal of at least one donor sheet or cover sheet, and are inherently incapable of continuous tone imaging.
Continuous tone imaging requires that image density be produced in proportion to the exposure energy received. Systems which meet this requirement include dye diffusion (or sublimation) transfer, and systems described in U.S. Pat. Nos. 4,826,976, 4,720,449, 4,960,901, 4,745,046, 4,602,263 and 4,720,450 wherein dyes (yellow, magenta or cyan) are created or destroyed in response to heat generated by laser exposure. These systems do not easily produce a neutral black colour or a high Dmax. Consequently, for medical imaging the main emphasis has been on systems involving the reduction of metal salts, especially silver salts, to the corresponding free metal.
Silver-based imaging elements that can be imagewise exposed by means of light or heat are well known. Silver halide conventional photographic and photothermographic elements are the most representative elements of the class of light-sensitive materials. In both conventional photographic (xe2x80x98wet silverxe2x80x99) and photothermographic (xe2x80x98dry silverxe2x80x99) elements, exposure of the silver halide in the photosensitive emulsion to light produces small clusters of silver atoms (AgO). The imagewise distribution of these clusters is known in the art as a latent image. Generally, the latent image formed is not visible by ordinary means and the photosensitive emulsion must be further processed to produce a visible image. In both dry and wet silver systems the visible image is produced by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms, i.e., the latent image. This produces a black and white image.
Conventional photographic silver halide elements require a wet development process to render the latent image visible. The wet chemistry used in this process requires special handling and disposal of the spent chemistry. The process equipment is large and requires special plumbing.
In photothermographic elements, the photographic silver halide is in catalytic proximity to a non-photosensitive, reducible silver source (e.g., silver behenate) so that when silver nuclei are generated by light exposure of the silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. The latent image is amplified and rendered visible by application of uniform heat across the element.
U.S. Pat. No. 5,041,369 describes a process that capitalizes on the advantage of a dry processed photothermographic element without the need for surface contact with a heating device. The photothermographic element is imagewise exposed with a laser which splits the beam using a second harmonic generation device. In this process, the element is simultaneously exposed with one wavelength of light and thermally activated by the second wavelength of light. Even though this process has the advantage of simultaneous exposure and heat development of the image, the equipment is complex and limited by laser outputs capable of generating two useful separate wavelengths.
Photosensitive emulsions which contain silver halide are well known in the art to be capable of causing high minimum density (Dmin) in both the visible and ultraviolet (UV) portions of the spectrum. The high UV Dmin is due to the inherent absorption in the near UV of silver halides, particularly silver bromide and silver iodide, and to high haze when silver halide and organic silver salts are present together. High UV Dmin is undesirable for graphic arts scanner and imagesetting films since it increases the exposure time required during contact exposure with other media such as UV printing plates, proofing films etc. High haze can also lead to loss of image resolution when imaged photothermographic elements are used as contact films. It is also well known that imaged photothermographic elements comprising silver halides are prone to unwanted build up of Dmin in the background areas, especially on prolonged exposure to light.
Closely related to the above-described photothermographic media are the materials described in U.S. Pat. No. 5,260,180, which discloses thermally imageable compositions comprising a silver salt of an organic acid, a reducing agent, and, optionally, an activator, coated together in a suitable binder, which can be rendered photoimageable by the addition of a tetrahydrocarbylborate salt. The compositions develop a black silver image when subjected to imagewise light exposure and uniform thermal development. It is believed that a portion of the silver salt is converted to the silver tetrahydrocarbylborate, which forms catalytic AgO cluster""s in response to light exposure. When a suitable sensitising dye is present, a laser may be used for the imagewise exposure.
Thermographic elements are a class of imaging elements that do not rely on silver halide based chemistry. They are commonly used in labels, tickets, charts for recording the output of medical or scientific monitoring apparatus, facsimile paper, and the like. In their most common form, thermographic elements comprise a support carrying a coating of a thermally-sensitive composition comprising a colour former and a developer which react together to generate image density on application of heat. Examples of colour formers include leuco dyes which may be oxidised to the corresponding coloured dyes by suitable developing agents. Mixtures of leuco dyes may give rise to a black image, but an alternative route to a black image is the thermal reduction (to the free metal) of a light-insensitive metal salt of an organic acid (especially a silver salt such as silver behenate) by means of a suitable reducing agent.
Conventionally, heat has been applied imagewise to thermographic elements by thermal print heads, thermal styli and the like. However, in recent years such materials have been adapted for laser address by incorporating in the thermosensitive coating one or more infrared (IR) absorbers. These compounds can absorb the output of IR lasers and thus generate heat in irradiated areas which triggers the thermographic chemistry. For example, U.S. Pat. No. 5,196,297 discloses recording materials which employ colour-forming di- and tri-arylmethane compounds possessing certain S-containing ring-closing moieties and a Lewis acid material capable of opening said moieties. The preferred Lewis acid is a silver salt such as silver behenate, which converts the colour-forming compounds to their coloured form under the action of heat. In some embodiments, the heat is supplied via absorption of laser radiation by an IR dye.
In the field of black and white imaging, EP-A-0,582,144 discloses a thermal recording material comprising a substrate coated with an imaging system, the imaging system containing (a) a thermally reducible source of silver, (b) a reducing agent for silver ion, (c) a dye which absorbs in the range 500-1100 nm, and (d) a polymeric binder. The material gives a black image in response to laser address without need for further processing, but the scan rates and dwell times quoted are impractically slow, e.g., 15 cm/sec and tens or hundreds of milliseconds respectively. Similarly, EP-A-0,599,369 discloses a recording material comprising a support and at least one imaging layer containing uniformly dispersed in a polymeric binder (1) a substantially light-insensitive silver salt in working relationship with (2) at least one organic reducing agent, characterized in that said organic reducing agent is a polyhydroxy spiro-bis-indane. In some embodiments, an IR absorber is also present and imaging is by laser address, but in the example given, a Dmax of only 0.47 was obtained and the writing time for an A3-sized image was 24 minutes. The imaging materials disclosed in both these patents are of the direct-write type, in which the image density is generated at the moment of laser exposure, and there is no capability for amplification via post-exposure processing.
EP-A-0,582,144 discloses placement of reducing agent in the same layer as the silver salt, whereas EP-A-0,599,369 discloses that placement of reducing agent in a separate layer is also possible, although no advantage is cited for this configuration, and indeed the Examples disclose only single-layer constructions. This accords with conventional wisdom regarding direct-write media imageable by laser address, where the generation of an adequate image density at a realistic scan rate is seen as the major problem to be overcome. Requiring the reducing agent to migrate from one layer to another before imaging can take place would be expected to increase the energy demand, and hence lower the writing speed.
WO95/07822 discloses imaging materials broadly similar to those of EP-A-0,599,369, except that additional restrictions are placed on the absorption spectrum of the IR absorber (in the interests of improved UV and visible transparency), and a wider range of reducing agents are described.
None of EP-A-0,582,144, EP-A-0,599,369 and WO95/07822 teaches any particular importance for the selection of the binders used, and all three recite a wide variety of polymers as being suitable. However, in the Examples of all these publications, polyvinyl butyral) is the only binder material disclosed for the silver-containing layers. Poly(vinyl butyral) has a glass transition temperature (Tg) of about 50xc2x0-56xc2x0 C.
U.S. Pat. No. 5,766,828 describes an IR laser addressable imaging element comprising: a substrate; a first layer comprising a reducible light-insensitive silver salt and a binder; and a second layer comprising an infrared absorber, a reducing agent for said silver salt and a binder; characterised in that the binder of said first layer is a polymeric medium having a glass transition temperature of at least 80xc2x0 C. Imaging elements are of the single sheet type, in which a single support sheet carries all the component layers. Apart from an optional heat treatment, no processing steps (such as wet development, peeling apart etc.) are required subsequent to laser imaging for the purposes of developing or fixing the image. The invention asserts that two-layer direct-write media are capable of high sensitivity, and that the two-layer configuration enables post-exposure thermal amplification of the image (which further enhances the sensitivity) and continuous tone imaging, neither of which is described in the prior art. Furthermore, the performance improves with increasing binder Tg which is contrary to expectations. The invention further extends to imaging methods employing such elements, comprising the steps of: 1) image-wise irradiating the element with IR laser radiation of sufficient intensity so as to generate a latent image of silver specks having a Dmax of less than 1.0, and 2) heating the element to produce a visible image having a Dmax of at least 2.5. This produces a monochrome silver metal image in response to laser irradiation, either directly or after uniform thermal processing.
A chemical composition according to the invention is capable of switching from a hydrophilic state to a hydrophobic state when heated, preferably by a focused IR laser. This composition also changes from a more water-soluble to a less soluble composition when heated. The degree of solubility and the degree of hydrophilic activity can be controlled over a wide range by mixing the composition with different polymers. Such compositions are of great commercial importance in the field of lithographic offset printing, which is based on the fact that the hydrophilic areas of an image will not carry ink. The making of lithographic printing masters is well known, however most lithographic masters require processing after exposure. The current invention allows lithographic masters, such as printing plates, to be used immediately after exposure without requiring any chemical development. The invention also enables the use of the composition to coat printing cylinders directly and image them on the printing press. Prior art thermosensitive composition based on physical effects (melting) or different reaction do not produce as sharp a switch of properties as the present invention. In this disclosure the term xe2x80x9cwater solubilityxe2x80x9d refers not only to true solubility but to the ability to be washed away by water or water-based solutions (aqueous solutions with or without organic solvents, alkaline agents, surfactants, etc.), even if the removal mechanism is based on effects other than true solubility in pure water. Other physical effects and phenomena such as softening, swelling, lifting and the like that assist in the differential removability of the layer after thermal exposure are included in the term xe2x80x9csolubilityxe2x80x9d.
In accordance with the invention, a water-soluble polymer is made to react with a metallic salt of a long chain fatty acid. As long as the mixture is not heated it is hydrophilic due to the water-soluble polymer. After heating, the water-soluble polymer reacts with the metallic salt to form a highly hydrophobic and insoluble polymer. While it is believed to be the nature of the reaction the invention should not be constrained by any explanation used in the disclosure. In order to make the composition compatible with imagewise heating using lasers, an absorber for the specific laser wavelength used has to be added. Absorbers can be broadband (covering a wide range of wavelengths) such as carbon powder or dyes tuned to a specific laser wavelength, such as IR absorbing dyes tuned to laser diodes.
In the most basic form the invention contains only these three ingredients (water soluble polymer, salt of fatty acid and laser absorber). In this form the unexposed areas are both hydrophilic and highly water-soluble. After heating with a laser, the exposed areas become highly hydrophobic and insoluble. In this form the invention is useful for making lithographic printing plates by coating a lithographic metal, such as anodized aluminum, with the composition. The unexposed areas are washed away and the exposed metal repels ink by carrying water.
A more useful form of the invention results when additional polymers and fillers are introduced to control the solubility of the unexposed areas without degrading the basic switch from hydrophilic to hydrophobic. For example, if a sufficient amount of polyvinyl butyral is added the unexposed areas are hydrophilic but not easily soluble, thus a printing master which does not rely on lithographic metal is created. Such a printing master has major advantages for making low lost lithographic plates. It can be coated on almost any substrate including re-usable lithographic masters, as old coating can be washed off after printing and a new coating applied without particular concern for contamination remaining on the substrate. Such material are also known as xe2x80x9csurface switchable polymersxe2x80x9d or xe2x80x9cswitchable polymersxe2x80x9d. An example of such a polymer is given in U.S. Pat. No. 4,081,572.